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World’s Smallest Semiconductor Laser Created August 31, 2009
Researchers at the Univ. of California, Berkeley, have created the world's smallest semiconductor laser, capable of generating visible light in a space smaller than a single protein molecule.
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This breakthrough, described in the online journal Nature, breaks new ground in the field of optics. The UC Berkeley team not only successfully squeezed light into a tight space, but found a novel way to keep that light energy from dissipating, thereby achieving laser action.
"This work shatters traditional notions of laser limits, and makes a major advance toward applications in the biomedical, communications and computing fields," said Xiang Zhang, Director of Berkeley's Nanoscale Science and Engineering Center.
The achievement helps enable the development of nanolasers that can probe, manipulate and characterize DNA molecules; faster optics-based telecommunications; and optical computing.
While it’s traditionally accepted that an electromagnetic wave-including laser light-cannot be focused beyond the size of half its wavelength, research teams around the world have found a way to compress light down to dozens of nanometers by binding it to the electrons that oscillate collectively at the surface of metals, an interaction known as surface plasmons.
Scientists have been racing to construct surface plasmon lasers that can sustain and utilize these tiny optical excitations. However, the resistance inherent in metals causes these surface plasmons to dissipate almost immediately after being generated, posing a critical challenge to achieving the buildup of the electromagnetic field necessary for lasing.
Zhang and his research team took a novel approach to stem the loss of light energy by pairing a cadmium sulfide nanowire with a silver surface separated by an insulating gap of only 5 nm. In this structure, the gap region stores light within an area 20 times smaller than its wavelength. Because light energy is largely stored in this tiny non-metallic gap, loss is significantly diminished.
With the loss finally under control through this unique "hybrid" design, the researchers could then work on amplifying the light.
"When you are working at such small scales, you do not have much space to play around with," said Rupert Oulton, the research associate in Zhang's lab who first theorized this approach last year and the study's co-lead author. "In our design, the nanowire acts as both a confinement mechanism and an amplifier. It's pulling double duty."
Trapping and sustaining light in radically tight quarters creates such extreme conditions that the interaction of light and matter is strongly altered. An increase in the spontaneous emission rate of light is a telltale sign of this altered interaction; the researchers measured a six-fold increase in the spontaneous emission rate of light in a 5 nm gap.
The researchers used semiconductor materials and fabrication technologies that are commonly employed in modern electronics manufacturing. By engineering hybrid surface plasmons in the tiny gap between semiconductors and metals, they were able to sustain the strongly confined light long enough that its oscillations stabilized into the coherent state that is a key characteristic of a laser.
Source: Univ. of California
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